Modern communication systems process quadradrature phase shift keyed (QPSK) signals, and other types of quadrature amplitude modulated (QAM) signals, that have high symbol rates. For example, symbol rates within the QPSK signals of code domain multiple access (CDMA) systems presently exceed one million symbols per second. At these high symbol rates, accurately identifying the carrier frequencies of the modulated signals is difficult.
Due to the difficulty in identifying carrier frequencies, measurement instruments that monitor code domain power (CDP), modulation accuracy and other critical performance measures of communication systems rely on estimates that the carrier frequency of the modulated signals fall within a specified offset range relative to a target carrier frequency. However, when the carrier frequency falls outside the specified offset range, the measurement instruments are unable to accurately and reliably monitor the communication systems. For presently available measurement instruments, this specified offset range is as narrow as +/xe2x88x921 KHz, making these measurement instruments unsuitable for monitoring base stations, mobile phones and other portions of communication systems in which the carrier frequencies deviate from the target carrier frequencies by large frequency offsets. Were estimates of carrier frequency accurately made, compensation for the large frequency offsets would enable the measurement instruments to accurately and reliably monitor communication systems.
Accordingly, there is a need for a system that accurately estimates carrier frequencies of QAM signals having high symbol ratesxe2x80x94even when the carrier frequency occurs at large frequency offsets relative to the specified target frequency.
Systems constructed according to the preferred embodiments of the present invention accurately estimate carrier frequency of quadrature amplitude modulated (QAM) signals that have high symbol rates.
A first system, constructed according to the first preferred embodiment of the present invention, includes a converter, complex signal generator, exponentiator, FFT module and processor to estimate carrier frequency of a QAM signal relative to a target carrier frequency. The converter converts the applied QAM signal to an intermediate frequency (IF) signal and digitizes the IF signal at a predesignated sample rate. The complex signal generator receives the digitized IF signal, extracts a real component of the digitized IF signal and performs a Hilbert Transform on the extracted real component to provide an imaginary component of the digitized IF signal. The complex signal generator then combines the real component and the imaginary component in a complex signal. The exponentiator, raises the complex signal to a predesignated power and the FFT module performs a Fast Fourier Transform (FFT), having a predetermined number of samples, on the complex signal as raised to the predesignated power. The processor, takes the magnitude of the resulting FFT, extracts the index of a sample within the resulting FFT having the greatest magnitude and estimates the carrier frequency of the applied QAM signal relative to a target carrier frequency based on the predesignated power, the extracted index and the predesignated sample rate.
A second system, constructed according to the second preferred embodiment of the present invention, includes a converter, a quadrature downconverter, an exponentiator, FFT module and processor to estimate carrier frequency of a QAM signal relative to a target carrier frequency. The converter converts the QAM signal to an IF signal and digitizes the IF signal. The quadrature downconverter, receives the digitized IF signal, extracts a baseband in-phase component of the digitized IF signal, extracts a baseband quadrature component of the digitized IF signal and generates a complex signal from the baseband in-phase component and the baseband quadrature component. The exponentiator, raises the complex signal to a predesignated power and the FFT module, performs a FFT, having a predetermined number of samples, on the complex signal as raised to the predesignated power. The processor takes the magnitude of the resulting FFT, extracts the index of a sample within the resulting FFT having the greatest magnitude and estimates the carrier frequency of the applied QAM signal relative to a target carrier frequency based on the predesignated power, the extracted index and the predesignated sample rate.